It is known that decorative laminates and pieces of furniture can be produced by coating particle boards with impregnated decorative papers. A crucial role is played by the impregnated paper, which is obtained by impregnating a basic paper with a specific impregnation solution or an impregnation resin. The impregnated paper is identified in this instance as pre-impregnated product and serves also as decoration-carrying component in the named applications.
A method of the initially described type is apparent from EP 0 223 922. This patent describes method for manufacture of widths of paper impregnated with synthetic resins in the form of solutions and dispersions. As impregnation fluid, a mixture is used of watery anionic copolymer dispersions on basis of acrylic acid, acrylic acid ester, acrylic nitrile, vinyl acetate and/or styrol and of watery anionic solutions of copolymerisates on basis of maleic anhydride or maleic acid with styrol, acrylic acid and acrylic acid esters.
WO 94/00523 discloses a watery impregnation solution, with which papers are impregnated, which are then used to manufacture laminated boards. The watery impregnation solution contains 5 to 90 parts by weight of polyvinyl alcohol on 10 to 95 parts by weight of a dispersion of an ethyl-styrol/acrylate/butyl-acrylate-copolymer.
According to the state of the art, since the end of the eighties, a pre-impregnated product has been known, from marketing efforts, which is obtained by impregnating a basic paper of a cellulose mixture of long- and short fibers with an impregnation solution known according to internal identification R20. The impregnated solution contains 15% of a glyoxal/urea-precondensate or -resin in watery solution, 57% of a 50% watery dispersion of a copolymer on basis of n-butylacrylate and styrol and 28% water. The pre-impregnate has various desirable properties, such as for example, high resistance toward hardening and abrasion, high temperature resistance, color-fast property, neutral natural color, excellent resistance to water, as well as excellent resistance to chemicals, high degree of flame-resistance, high optical transparency, is devoid of odor and taste, as well as toxicologically completely harmless. Specifically involved is a low formaldehyde pre-impregnate. However, when unfavorable temperatures and varnish systems are combined, it is not sufficiently stable with respect to turning yellow.
The term "turning yellow" shall not mean, in this case, insufficient light resistance, but the chemical reaction which occurs based on the employed impregnation solution. The known pre-impregnate finds little application in the so-called "white" sector due to said yellowing. The skilled person understands by "white" sector, light-colored laminates or pieces of furniture, where turning yellow has a particularly detrimental effect.